The optical telecommunication field has undergone very rapid development, to the extent that commercial fiber-based systems are now being installed routinely. However, the technology is still experiencing major shifts in direction.
For instance, whereas first generation fiber systems almost invariably were designed to operate at a wavelength of about 0.8 .mu.m, there is now growing interest in systems having an operating wavelength of about 1.3 .mu.m, to take advantage of the loss window that exists in silica-based fiberguide in that wavelength region. Even lower loss is obtainable at about 1.55 .mu.m. It is expected that some future optical transmission systems will operate at that wavelength, pending the development of economical sources and other components for use at 1.55 .mu.m.
A further example of a major shift in the direction of the technology is the change from multimode (MM) to single mode (SM) technology. This change is primarily driven by the demand of the marketplace for higher and higher bandwidths.
As is well known to the practitioner, optical fibers can be designed to either support a multiplicity of guided modes of electromagnetic radiation at the design wavelength, or to support only a single guided mode at that wavelength. For background on this as well as other aspects of optical fiber telecommunications, see, for instance, Optical Fiber Telecommunications, S. E. Miller and A. G. Chynoweth, editors, Academic Press, 1979, especially pages 38-53.
MM fibers typically have a fiber core diameter that is large compared to the operating wavelength of the fiber. The fiber core is the central region of the fiber in which the refractive index of the material is higher than in the surrounding fiber region, the cladding. A typical core diameter of MM fiber is about 50 .mu.m, with 125 .mu.m being a typical outside diameter of currently used fiber. Because of the relatively large size of the core, it is comparatively simple to assure core-to-core alignment when joining MM fibers, and a variety of MM connector designs are in use.
SM fiber theoretically can have much higher bandwidth than MM fiber, due, inter alia, to the absence of mode dispersion in the former. For that reason, SM fibers are advantageous for long-haul high transmission rate applications. SM fiber for use at 1.3 .mu.m has been developed, is commercially available, and is being installed.
SM fibers have a core diameter that is typically much smaller than that of MM fiber. At this time, the diameter of a typical exemplary SM fiber core is about 9 .mu.m. In order to achieve the low splice loss desirable in long-haul communications links, it is typically necessary to align the two abutting cores of SM fibers to within less than about one micrometer. Such joints currently generally require active alignment of the cores, e.g., alignment while monitoring the amount of radiation lost at the junction or transmitted into the receiving fiber.
Optical fiber is currently almost invariably installed in the form of multifiber cables. For connection of a single user installation to the loop, the cable may be a simple one, containing as few as two fibers. On the one hand, in loop or trunk applications or the like, the cables typically have a much higher fiber count.
Several types of optical fiber cables have been developed, including loose tube and other stranded fiber cables, and ribbon cable. For a description of a loose tube cable see, for instance, D. Lawrence and P. Bark, International Wire and Cable Symposium Proceedings, 1983, pp. 301-307. Stranded fiber cable designs have been found suitable for MM as well as for SM fiber, and SM fiber cable of this type, including high fiber count cable, is commercially available. Indeed, Lawrence et al report that other cable types have been found unsuitable for high fiber count SM fiber cable.
Other variants of stranded cable are exemplified by U.S. patent application Ser. No. 518,145, filed July 28, 1983, now U.S. Pat. No. 4,645,298, and by U.S. Pat. No. 4,361,381. The former shows fibers stranded onto a strength member within a loose tube, and the latter is an example of a slotted core cable.
Currently available MM fiber ribbon cable comprises one or more fiber ribbons, with each ribbon comprising a multiplicity of fibers (presently typically 12) embedded in a thin, flexible supporting medium. See, for instance, R. D. Standley, The Bell System Technical Journal, Vol. 53, pp. 1183-1185 (1974). See also U.S. Pat. No. 4,078,853, issued Mar. 14, 1978 to R. A. Kempf et al, as well as U.S. Pat. No. 3,937,559, issued Feb. 10, 1976, and U.S. Pat. No. 4,110,001, issued Aug. 29, 1978. Commercially available ribbon cables contain up to 12 ribbons, yielding a total fiber count of up to 144 fibers. This, of course, is not necessarily the highest fiber count achievable with ribbon cables, and the count could be increased if this appears indicated.
MM fiber ribbon cable, the only type of ribbon cable known to the prior art, has a number of attractive features. One of the most important is its potential for array connectorization. Array connectors are well known to those skilled in the art. See, for instance, U.S. Pat. No. 3,864,018, incorporated herein by reference. Such connectors can be factory installed, and can drastically reduce the time required per fiber connection, as compared to single fiber joining techniques of the type commonly used with stranded cable.
A further advantageous feature of MM ribbon cable is its efficient use of duct or conduit space. For a given (relatively high) fiber count, a ribbon cable can have a significantly smaller outside diameter than a stranded cable, and therefore greater fiber density, than an equal fiber count stranded cable.
Despite these and other advantages of the optical fiber ribbon cable, the design has not been widely adopted by the industry. To the best of our knowledge, only one manufacturer has supplied significant quantities of MM ribbon cable. And the market share of ribbon cable has declined recently, due primarily to the increased use of SM fiber cable, which has exclusively been stranded cable.